1. Field of the Invention
This invention relates generally to unique aldehyde condensation resins, and more particularly to aldehyde condensation resins which are useful as adhesives and binders.
2. Description of the Prior Art
Aldehyde condensation resins, and in particular, phenol-aldehyde resins, are commonly used as adhesives in the formation of plywood, particle board and hardboard.
One difficulty which the aldehyde resin adhesive industry has been concerned with is the prevention of dry-out of the adhesive during assembly of the contacted pieces being bonded. In the plywood industry in particular, the aldehyde resin adhesives are usually applied in the form of a viscous, water containing mixture or solution, or in some cases, in the form of an alcoholic or other hydrophilic solution. There is a tendency however, for many materials to absorb or dessicate the water or solvent medium, which results in poor adhesive bonding with the adjacently applied substrates. Recently, this problem has become accentuated by the increased use of southern pine woods in plywood formation. Such woods, however, generally possess highly absorbent characteristics. Normally, a lag time of 10 - 25 minutes will occur between the time the adhesive is applied to the individual veneers forming the plywood panel layup, and the time that heat and pressure are applied to convert the aldehyde resin to its "C" stage or thermoset and cured condition. During this lag, or assembly time, most presently commercial water containing aldehyde adhesives will become at least partially dessicated.
Efforts have been made in the prior art to reduce this bond dryout tendency, but no completely satisfactory solution of the problem has been reported. Most prior art attempts have concentrated on varying the quantity of catalyst used in forming the resin, or in varying the ratios of resin components. For instance, in phenol-formaldehyde resins, which are the most common resin used in such adhesives, efforts have been made to solve the bond dryout problem by varying the sodium hydroxide content, or by varying the phenol/formaldehyde ratios. However, such efforts have largely resulted in the formation of resins which are either too easily dried out and hence form quite brittle adhesive layers, or which have excessive flow characteristics hence causing the resin to easily flow from the bonding site and perhaps be too slow in curing as well.
U.S. Pat. No. 1,960,176 disclosed procedures for preparing a phenol-formaldehyde resin solution which could be impregnated into tissue paper and dried to produce a supported dry bonding film glueline for making hot pressed laminated products from wood veneers. This resin is indicated as having the property of thermoplasticity which can be set to a hard infusible, permanent form at temperatures of about 130.degree. C. The complexity of the total preparation and slowness of the cure mitigates against its direct use as a plywood adhesive.
Efforts at physically combining a high thermosetting phenol-formaldehyde resin, with high thermoplastic phenol-formaldehyde resins have also not been fully successful, since such combinations have resulted in poor or inadequate adhesive bonding. For instance, when adhesives such as reported in U.S. Pat. No. 2,456,006, and U.S. Pat. No. 2,631,097 in which more or less thermosetting and thermoplastic resins are reportedly used in combination, the quality of the adhesive bond was not found to be superior to homopolymers of comparable composition.
It was also suggested to prevent bond dryout by incorporation of viscosity enhancers, such as starch into the adhesive (U.S. Pat. No. 3,544,493), however, such efforts have also not proven to be fully satisfactory.
Accordingly, a need continues to exist for an adhesive which will circumvent the bond dry-out problem, even during long periods of assembly time.